This invention relates to time division (TD) multiplexing of a plurality of digital video signals for transmission through a digital communication network. In particular, the digital communication network is an optical network between studios of a television broadcasting station.
Use of the optical network is discussed in an article written by T. Kajitani and two others and published 1991 in Japanese in the Transactions of Optical Communication System Study Group, No. OCS 90-65, of the Institute of Electronics and Information Communication Engineers of Japan, pages 39 to 64, under the title of "Video Signal Distribution System for Broadcasting Stations Utilizing Photonic Wavelength-division and Time-division Hybrid Multiplexed Network" as translated by the authors into English. The authors include the present inventor as a first one of the two others.
A multiplexing system for such video signals is disclosed in Japanese Patent Prepublication (A) No. 268,581 of 1993. In the multiplexing system, each video signal is first scrambled into a scrambled signal comprising, in each channel, frame bit positions where a frame pattern and a channel identification code should be inserted. At the frame bit positions of one of a plurality of channels, actually overwritten are the frame pattern and the identification code indicative of the channel under consideration.
Such scrambling of a plurality of digital bit streams was already discussed in an article contributed by Doowhan Choi to the AT&T Technical Journal, Volume 65 (1986), Issue 5 (September/October), pages 123 to 136, under the title of "Parallel Scrambling Techniques for Digital Multiplexers". Scramblers are exemplified, which have a prescribed number of scrambling stages greater than the number of the digital bit streams. Furthermore, this article deals with scramblers where the prescribed number is equal to or less than the number of digital bit streams.
In connection with the foregoing, it is possible to understand as will later be described in greater detail that a conventional time division multiplexing system for a preselected number of input digital video signals comprises a time division multiplexer for time division multiplexing the video signals bit by bit into a multiplexed signal. Each video signal has a tributary transmission rate between about 150 and 300 Mb/s. The multiplexed signal has a multiplexed transmission rate between about 1 and 3 Gb/s. The multiplexed signal is therefore transmitted through an optical network as well as the input digital video signals in the manner described at the outset.
When produced by the conventional time division multiplexing system, the multiplexed signal may comprise, as a long-continued pathological pattern, a very long succession of a common bit polarity, such as consecutive zeros or ones. The long pathological pattern results in a fear of occurrence of transmission errors. Such undesirable phenomena become severe when each video signal is a digital serial video signal in which a considerably long continuation of zero or one bits frequently appears. The long continuation is, for example, either alternate successions of twenty one bits and twenty zero bits or a succession of two ones followed by nineteen zeros. A maximum bit length of about seventy bits of such a same polarity is therefore allowed in the multiplexed signal for transmission through the optical network in order to reduce transmission errors.